Predictive maintenance of ac induction motors typically involves many complex and interrelated calculations of various motor parameters based on readings acquired from sensors attached to the motor. For some calculations, the accuracy of the motor parameter calculation is dependent upon an accurate determination of motor speed. For example, it is known that the speed of an ac induction motor will vary in relation to loading of the motor. Accordingly, a typical calculation of motor load involves determining motor speed and then calculating load from speed. As the accuracy of the speed determination decreases, so too does the accuracy of the load calculation.
As another example, trend parameters acquired from flux spectra commonly are associated with spectral peaks (such as running speed sidebands) that lie a significant distance away from the primary running speed peak. An error introduced in the determination of running speed is amplified as the analysis moves out in the frequency spectrum away from the primary running speed peak. The result of this amplification of the running speed error is that it often becomes very difficult to locate a particular peak of interest even when the running speed error is small. Unfortunately, current methods for determining the speed of ac induction motors do not provide the level of accuracy often needed by the analyst to eliminate these problems.
Therefore, there is a need for a highly accurate and reliable method and apparatus for determining the speed of an electric motor.